The overall objective of the proposed study is to understand the structural basis for molecular recognition and assembly between TGF-beta family ligands and cell surface receptors. The TGF-beta ligands exert a wide range of biological functions. Two types of receptors involved in the signaling cascade, each with a single transmembrane helix and a cytoplasmic Ser/Thr kinase domain, form a large protein complex with the ligand to initiate the signaling process. Our specific goal is to have a structural understanding on how the binding of ligands to the extracellular domain of the first receptor, and its subsequent complex with the other receptor transduces the message to the downstream components of the signaling pathway. Bone morphogenetic proteins (BMP) is a subfamily of TGF-beta ligand superfamily. We will focus on Activin/BMP signaling system to address these questions. In addition, we will study the Activin/BMP interaction with its antagonist Noggin. We will study structures of ligand-binding domain of inhibin receptor. In this study, our primary focus is two-fold: 1) to decode fundamental structural determinants specific and common among those ligands, and 2) to understand conformational changes in the ligand induced upon complex formation with respect to the biological signaling outputs. Based on stoichiometric ratios and binding affinity of different ligand/receptor complexes, we will establish the thermodynamic and kinetic rates of the processes. Additionally, we will determine structures of BMP variants in complex with Noggin. I believe results from these studies will provide a firm basis to understand structural basis for molecular recognition and receptor assembly, from which we can facilitate the design of effective therapeutic means to be used in modulating various hormone-related and neurological diseases.